Method of inhibiting corrosion



3,091,591 METHQD OF INHIBITLNG CORROSION Norman Hackerman and Robert R. Armand, Austin, Tex., assignors to Texas Research Associates Corporation, a corporation of Texas No Drawing. Filed May 16, 1960, Ser, No. 29,138 9 Claims. (Cl. 252148) The present invention relates to a new and improved corrosion inhibitor for ferrous metals and more particularly to a corrosion inhibitor suitable for use directly in a corrosive medium either as a solution or as an intimate mixture.

It is a principal object of this invention to provide a new and improved composition which will retard or inhibit the corrosion of ferrous metals in aqueous salt solutions, in hot concentrated acids, or in other environments in which corrosion of such ferrous metals is likely to occur.

Another object of the present invention is to provide a new and improved composition which may be economically produced and quickly and easily added to a corrodant whereby the corrosion of any ferrous metals in such corrodant is substantially reduced.

Still another object of the present invention is to provide a new and improved method of making a corrosion inhibitor whereby a cyclic imine is added to a solution.

A further object of the present invention is to provide a new and improved corrosion inhibitor which may be applied to a metal surface by spraying, dipping or painting whereby such corrosion inhibitor .adsorbs on the metal surface to provide a protective layer thereon.

A preferred embodiment of this invention will be described hereinafter, together with other features thereof, and additional objects, features and advantages of the invention will become evident from such description.

Briefly this invention relates to compounds which may be added directly to or intimately mixed with a variety of corrodants such as aqueous salt solutions, hot concentrated acids or other corrosive environments in which ferrous metals might be deposited and which substantially inhibit the corrosion of such ferrous metals deposited therein.

Such preferred compounds or corrosion inhibitors are the cyclic saturated imines where It varies from six to twelve, plus the substitution products where the hydrogen atoms .are replaced asby alkyl groups. These compounds are most suitable for use directly in a corrosive medium and when addedwill form either as a solution with the corrosive medium or an intimate mixture therewith. These compounds may also be applied directly to a metal surface by spraying, dipping, or painting since they act as corrosion inhibitors by adsorbing on the metal surface to form a protective layer thereon. .However, as this protective layer is probably only a few molecules thick, optimum protection is obtained by the direct addition of the imine or sulfide compound to the corrodant solution so that in any of the places where the protective layer may become displaced, new imine molecules present in the solution quickly take the place of those molecules previously displaced. Such adsorption from the corrosive solution takes place in preference to corrosion by the corrosive media and a protective layer begins to build up as soon as the inhibited corrodant contacts the metal surface, which rapidly educes the rate of corrosion to a minimum. The length of time required for the protective layer of inhibitor to form on the metal surface is relatively short, and depends 3,91,591 Patented May 28, 1963 on the concentration of the inhibitor, whether or not such inhibitor is completely in solution, the concentration of the corrodant, and the temperature of the whole system.

The cyclic saturated imines substantially inhibit corrosion of ferrous metals submerged in the following corrodants: brine solutions; solutions up to commercial strength of such acids as hydrochloric, perchloric, acetic, propionic, and formic; sulphuric and phosphoric acid up to at least 12 N; and non-oxidizing acids in general.

The corrosive action of strong acids such as hydrochloric acid is inhibited by these inhibiting compounds just as readily as the corrosive action of weak acids such as acetic acid. It should be noted however, that the use of imines as inhibitors is restricted to use in an enviroment of a non-oxidizing corrodant. For example, in a solution of concentrated nitric acid, the inhibitors would be rapidly destroyed. Oxidation of the inhibitor would also occur in hot, 98% sulfuric acid (36 N). The inhibitors as stated above function properly in 12 N sulphuric acid. The terms corrodant .and corrosive solution as used in the claims hereinafter mean a non-oxidizing corrodant or corrosive solution.

The optimum concentration of the corrosion inhibitor is dependent upon the concentration of the corrodant e.g. a strong concentrated acid solution requires a higher concentration of inhibitor than a less concentrated acid solu tion to produce the same degree of corrosion inhibition. Where the corrosion inhibitor is a cyclic saturated imine such as hexamethylenimine, heptamethylenimine, octamethylenimine, nonamethylenimine, or decamethylenimine the preferred concentration of the inhibitor ranges from approximately about .001% to about .25 for a solution which is neutral or which has a low concentration of acid and from about 1% to about 8% for a solution which has a high concentration of acid.

The upper range figures above represent values approximately limited by the solubility of the inhibitor or compound. However, increased protection can be obtained by an intimate mixture of the compound with the corrodant (e.g. an emulsion) where the compound or inhibitor is present in an amount exceeding its solubility.

Testing procedure was as follows:

Boiling temperature acid.--Measurements were made with coupons of steel machined from /1" rounds of 1020* steel to give an area of 4.64 square centimeters. An apparatus was assembled such that the vapors from a boiling solution of hydrochloric acid were continually condensed and returned to the bulk acid solution. Various imines from hexamethylenimine to decamethylenimine were added to the acid in several concentrations. The coupons above were placed in the boiling hydrochloric acid and imine solution using one coupon for each specific test. As the composition of the acid solution had previously been adjusted to give a constant boiling solution and as imines dissolve to give non-volatile salts, the composition of the solution surrounding the coupon changed only as a result of dissolution of iron and evolution of hydrogen.

Two types of measurements were'made. (1) Gas evolution measurement-the hydrogen evolved during exposure of the coupon to the acid solution was passed over to a gas burette and its volume taken as a function of time. The corrosion rate (mg./dm. day) was calculated from the slope of a plot of milliliters of H evolved versus time. That portion of the curve covering the time period from 5 to 25 minutes was taken for calculation of the rate which was obtained by means of the multiplication factor, 773x10 (mg.-min./ml.-dm. -day); (2) Weight loss measurements-the weight lost to the solution by a coupon of known dimensions during the course of one hour was determined by weighing the coupon before exposure to the boiling acid solution and again following exposure. After subsequent suitable washing and drying a corrosion rate (mdd) was calculated by multiplying grams lost for the 60 minute period by 5.17 1O (hr.-mg./dayg.-dm.

Room temperature acid.-A standard polarization technique was used such that an iron wire (Mallinbrodt, 99.9% Fe) of 0.25 cm? area was exposed to a solution of the imine in constant boiling hydrochloric acid (20% HCl) and simultaneously connected to a battery and a microammeter (the polarizing circuit) as well as an elec trometer (the potential measuring circuit). The potential of the wire was measured with respect to a saturated calomel reference electrode for various current levels. This procedure was followed with the wire connected to the positive pole of the battery (anodic) and also with the wire connected to the negative pole of the battery (cathodic). The data obtained was plotted as potential versus current, and by suitable short extrapolations from the Tafel region of the resulting curves, a quantity was derived known as the exchange current or corrosion current. This current is related to the corrosion rate by the constant factor 10.0 (mg./day- .ta.-dm. which arises in the conversion of units from (micro) coulombs/sec. to mg./ dm. day. The room temperature data was recorded graphically as a function of inhibitor concentration (g./l00 ml.).

For illustrative purposes only and in no way intended to limit the scope of the invention some examples of tests conducted are as follows:

EXAMPLE 1 Corrosion Rates by Gas Evolution Measarments From Constant Boiling HCl at 107 C.

Corrosion Rates by Weight Loss in One Hour in Constant Boiling HCl at 107 C.

Rate (mdd. X

Percent Inhibited Inhibitor Uninhibited Hexamethylenimme 4;. EXAMPLE 3 Corrosion Rates From Polarization Measurements in Constant Boiling HCl at Room Temperature In addition to the test set forth above, steel powder was placed in solutions of 1 N sulfuric acid and hexamethylenimine and also in solutions of 1 N sulphuric acid and hexamethylene sulfide. Both types of solutions were maintained at room temperature and there was no measurable hydrogen evolution in two days. The corrosion of the steel powder was reduced essentially to Zero.

From the data recorded hereinabove it can easily be seen that the addition of the cyclic saturated imines to a corrodant such as a strong acid substantially inhibits the corrosion of ferrous metals in contact with such corrodant.

Broadly the invention relates to a new and improved corrosion inhibitor which may be directly added to a nonoxidizing corrodant to substantially inhibit corrosion of ferrous metals in contact therewith.

What is claimed is:

1. A method for inhibiting the corrosion of ferrous metals in non-oxidizing alkaline and acidic corrodant systems, characterized by contacting such metals with at least one saturated cyclic imine of the formula (OHDH NH where n is an integer from 6 to 12.

2. The method of claim 1 wherein said at least one imine is applied to the surface of the metal prior to contact of the metal with the corrodant system.

3. The method of claim 1 wherein said at least one imine is incorporated in the corrodant.

4. The method of claim 1 wherein imine is hexamethylenimine.

5. The method of claim 1 imine is heptamethylenimine.

6. The method of claim 1 imine is octamethylenimine.

7. The method of claim 1 imine is nonamethylenimine.

8. The method of claim 1 imine is decamethylenimine.

9. The method of claim 1 wherein imine is an alkyl substituted imine.

said at least one wherein said at least one wherein said at least one wherein said at least one wherein said at least one said at least one References Cited in the file of this patent UNITED STATES PATENTS 2,160,915 Schreiber June 6, 1939 2,776,263 Hiskey et a1 Jan. 1, 1957 2,850,461 Bloch et al. Sept. 2, 1958 2,981,617 Hager et al. Apr. 25, 1961 OTHER REFERENCES Ruzicka et al.: Uielgliedrige heterocyclische Verbindungen, Helvetica Chimica Acta (1949), pp. 544-52. 

1. A METHOD FOR INHIBITING THE CORROSION OF FERROUS METALS IN NON-OXIDIZING ALKALINE AND ACIDIC CORRODANT SYSTEMS, CHARACTERIZED BY CONTACTING SUCH METALS WITH AT LEAST ONE SATURATED CYCLIC IMINE OF THE FORMULA 